This invention relates to thermally efficient insulating blankets for helium cooled superconducting magnet assemblies for magnetic resonance imaging (hereinafter called "MRI"), and more particularly to an improved multilayer wide blanket suitable for MRI systems utilizing a recondenser for recondensing the resultant helium gas back into liquid helium.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing a cryogen such as liquid helium. The extreme cold provided by the boiling helium maintains current flow through the magnet coils after a power source initially connected to the coil (for a relatively short period) is disconnected due to the absence of electrical resistance in the cold magnet coils, thereby maintaining a strong magnetic field. Superconducting magnet assemblies find wide application in the field of MRI.
The provision of a steady supply of liquid helium to MRI installations all over the world has proved to be difficult and costly leading to considerable research and development effort directed at minimizing the need to replenish the boiling liquid helium such as by recondensing and recycling the resultant helium gas. Also, it is desirable to avoid the difficulties encountered in storing the necessary reserve supply of liquid helium at cryogenic temperatures of around 4 K (or close to absolute zero) and the related problems of periodically transferring a portion of the liquid helium in the storage reservoir to the liquid helium supply in the MRI superconducting magnet.
In a typical MRI magnet, the main superconducting magnet coils are enclosed in a cylindrically shaped cryogen pressure vessel defining an imaging bore in the central region along its axis. A vacuum vessel surrounds the pressure vessel and a mechanical cryocooler in the space between the two vessels provides cooling to recondense the helium gas resulting from the boiling back to liquid helium for reintroduction and reuse in the helium vessel.
Superconducting magnets which recondense the helium gas back to liquid helium are often referred to as zero boiloff (ZBO) magnets. In such ZBO magnets, insulation is provided between the vacuum and helium pressure vessels and also around the recondenser between the recondenser and the pressure vessel to minimize heat loss. High efficiency insulation is very important in such ZBO systems since the mechanical cryocooler is often taxed to its cooling capabilities and heat losses can preclude proper or efficient operation of such systems. In addition, such insulation should provide at the same time a thermally efficient way for the escape of trapped air or gasses in the blanket during evacuation of the superconducting magnet.
MRI superconducting magnets are necessarily quite large to accommodate patients in the bore leading to the requirement for wide blankets to, for example, surround the pressure vessel which typically can have an outer diameter of 2 meters and a length of 2 meters. Sufficiently wide thermal insulating blankets are cumbersome to handle and difficult to manufacture.
However, highly reflective, very thin single or double side aluminized plastic film is commercially available in widths of only up to 60 inches. For use in MRI superconducting magnets, the width of multilayer insulating blankets required is frequently more than the 60 inches available, often as much as 80-100 inches wide. Moreover, the blankets have proven difficult to join without unacceptable heat loss at the joint between the multilayer insulating blankets. In addition, while thermal insulation and shielding is required of such blankets, it is also important to provide a path for the escape or evacuation of any trapped air or gasses in the insulating spacer layers interposed between the aluminized film layers.
Attempts to avoid unacceptable heat loss at joints have included spiral winding of blankets around cylindrical components such as the radiation shield in the magnet or configurations with interposed edges of adjacent reflective films resulting in a double thickness blanket at the overlapped seams.
Increased thickness of the multilayer blankets is disadvantageous in superconducting MRI magnets, especially in the warm patient bore, because the double thickness must be accommodated without MLI compression in a place where space is at a premium. If one-half inch of radius can be saved in the warm bore, the resulting smaller diameter superconducting coils use less superconducting wire, saving up to 4,000 feet of superconducting wire.